Low Power Regulators

[andybarrett1]

Hi Again

I have a few projects out in the field that are battery operated…. They all have a PP3 battery and a sleep function.

At the moment I am just using a 78l05 to drop the 9volts to the 5volts needed for the PIC… Am toying in the future of replacing with a LF (3.3v) version of PIC but at moment stuck with what I have.

A bigger drain on my battery than the PIC is the linear regulator…So am thinking of a replacement, at the moment this :-

http://www.ti.com/lit/ds/symlink/lm2936.pdf

Has anyone any better suggestions…. again open to other ideas before I go for broke .

Thank you for reading

Andy

[ruijc]

Hi there,

I was also there...

I have a few projects on my desk that are 9V battery powered and I was looking for a better replacement for the L7805.

I checked some references but the one I have chosen was the LP2950 which has the 5V and the 3.3V versions.

I believe that the specs are better than the LM2936 for output current ( 100ma against the 50ma ), dropout voltage and others.

I don't know if there are better ones out there in the market but other guys can post their thoughts too.

Regards
Rui

[LinkMTech]

I've had pretty good results with the MCP1804.
50uA quiescent current.
Same drop out voltage at 20mA but about 1V higher than the LP2950 at 100mA.

[Acetronics2]

LExx from ST have pretty good performance too ...

Alain

[andybarrett1]

I've had pretty good results with the MCP1804.
50uA quiescent current.
Same drop out voltage at 20mA but about 1V higher than the LP2950 at 100mA.

Just ordered some freebies from Microchip to Sample….

Ta Andy

[andybarrett1]

I've had pretty good results with the MCP1804.
50uA quiescent current.
Same drop out voltage at 20mA but about 1V higher than the LP2950 at 100mA.

Just ordered some freebies from ST to Sample….

Ta Andy

[andybarrett1]

Hi there,

I was also there...

I have a few projects on my desk that are 9V battery powered and I was looking for a better replacement for the L7805.

I checked some references but the one I have chosen was the LP2950 which has the 5V and the 3.3V versions.

I believe that the specs are better than the LM2936 for output current ( 100ma against the 50ma ), dropout voltage and others.

I don't know if there are better ones out there in the market but other guys can post their thoughts too.

Regards
Rui

Just ordered some freebies from ST to Sample….

Ta Andy

[ardhuru]

Am toying in the future of replacing with a LF (3.3v)

Andy

Since you're willing to operate the circuit on 3.3v, why not eliminate the PP3+regulator combo altogether, and go with a protected 18650 Lithium-Ion instead? You'll get a far longer runtime than a PP3+regulator, plus this is chargeable!

Regards,

Anand

[Charlie]

Well Anand, that may or may not be a good idea.

It depends on the circuit and application. First, the battery fully charged is 4.2V so if you have 3.3V parts that can't go over 3.6V (most of them) you will destroy your components. Then the battery voltage drops pretty linearly over time to about 2.75V when you should recharge it. If you are using the ADC, this change in voltage may mess up your measurements. There are other voltage dependent parameters that may or may not be critical to the application. Finally, these self discharge about 15% per month sitting on the shelf, so if the low power requirement comes from trying to make a battery last as long as possible, there are far better types of batteries - Lithium/Iron Disulfide for instance that have a 20 year shelf life and a much flatter discharge voltage. Without the actual application we can't be completely sure what's important.

But you are correct to consider going away from the 9V battery. Nearly half the power is currently being wasted as heat. 4 AAs (Li/FeS2) in conjunction with a low drop out and low idle current regulator would be a great generalized solution.

[andybarrett1]

Charlie and Ardhuru

Taking on board what you are both saying.... My problem is space as well as power consumption. At the moment my circuits are draining 0.860mA sleeping and 20mA (5v) when awake.... No ADC's yet.

2AA's is about as much space as I can afford...Given the really good LDO (0.2mV Typical) of the regulators mentioned I could in theory use 1 18650 Lithium-Ion Battery @3.7v 2000mA and still have 3.3mV Available as a regulated output if I was to drop to a LF Pic?

Thank you for reading

Andy

[Charlie]

Short answer is "it depends". What is your circuit / application?
Do you have any parts other than the PIC, and do they require 3.3V? Is your objective in low power to have batteries last a long time? If the answer to either of these is yes, 18650 is a bad choice.

[ardhuru]

Okay, here are a couple of ideas.

Is your project critical of the absolute value of the voltage? There are pics that operate from 2+ volts right upto 5, and anywhere in between.

In my opinion, if you use one of these you wouldnt need the LDO. The pic would happily chug along till the battery is cut off by its internal protection (which is why I's specified a 'protected 18650')

I've read that the 18650s typically loose 4 to 5% voltage on the shelf the first month after a complete charge, and about 2% every month thereafter. (Cant vouch for the exact figures). BUT, in my experience, gained from 18650s pulled out from a discarded laptop battery pack, they seem to be perfectly operational after months of non-use. So, a fully charged 18650 (which are very easy to charge within your projects as well, using modern chips like TP4056) will give you a very decent runtime, especially if comapared with a PP3+LDO circuit. Alternately, uou could use one of those cheap power banks used for cell phones to charge an 18650 from any USB source.

And lastly, if space is at a premium, there's a smaller sibling to the 18650; the 14500. About the size of a AA cell, 3.7 volts and 750 to 900 maHr, depending on the brand.

Regards,

Anand

[andybarrett1]

HI Anand.

In theory a great idea... BUT :- LCD and RTC has closer power supply tolerances than the PIC... ie +/-3.3v or 5v. So not as forgiving !

A decent lithium PP3 is also rated at around 900mAH so....Devil and a hard place or whatever the phrase is.

I think the LDO Regs are the way to go .... but still open to suggestions re the source of power.

Andy

[Ioannis]

Why not just use ONE 1,5 volt battery and then step up the voltage with this:

http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en547080&utm_source=MicroSolutions&utm_medium=&utm_term=&utm_content=&utm_campaign=MCP1640

or other similar step ups from Microchip that work downt to 0,35Volt?

And if you want really low active current of about 4uA but at the cost of 0,85volt of Vin, this is the best choice:

http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP16251

Ioannis

[Charlie]

I've read that the 18650s typically loose 4 to 5% voltage on the shelf the first month after a complete charge, and about 2% every month thereafter. (Cant vouch for the exact figures).

Anand
It's hard to find exact figures, but in this article http://www.powerstream.com/18650-high-discharge-rate.htm they quote a Samsung spec at nearly 10% self discharge per month. So maybe fine for your digital camera, but wrong choice for something like a remote sensor that can't be recovered or recharged for many months. Not all lithium technologies have the same attributes. There are some that have very stable output voltage and last for 20 years at low discharge rates. And there are some, like the one you keep promoting, that the voltage changes nearly 30% while it self discharges in well under a year. But you CAN recharge it - so in many applications, it's a better choice. That's why I keep asking about the application (and not getting an answer so I'll stop now). Ultimately there are many solutions each with advantages and disadvantages. If we keep visualizing different requirements, we will keep suggesting different solutions. Without a target, how do you know if you hit it?

[andybarrett1]

Charlie

Totally missed you asking (My Bad).

Application is 818, 1307RTC and 1620LCD. No Analogs. One vibration sensor DI as interrupt.

Wakes up on vibration. Displays time for 20Seconds Goes back to sleep.... Time stamp is recorded on remote DVD System by a camera.

At moment all running from a pp3 9volt .. LDO reg and 5volts. Looking to drop to 3.3v and use a better source of power.

Hope that explains enough ??

Thanks all for reading.

[Ioannis]

You said there is room for two AA batteries if I understood right. With just one battery and the following suggestion you will have more working time on 3.3 volts draining an AA alkaline really flat.

http://www.picbasic.co.uk/forum/showthread.php?t=19858&p=131836#post131836

Ioannis

[Charlie]

Both the RTC and LCD are 5V devices - RTC min is 4.5V, LCD min is 4.7 V.
So you really can't run your circuit at 3.3 V.
You will either need to use Ioannis' suggestion (which I also recommend), or go back to your 2936 regulator with at least a 6V battery. The 9V batteries have pretty low capacity - you might be better off with a couple 3V button cells; I have not worked out the details on those.

[AvionicsMaster1]

I'm just curious and was wondering:

MAX1620/MAX1621 states it can use a range of voltage between 3 to 5.5 volts.
DS1390 states it can do 2.97 to 5.5 volts.
16LF818 states it can do up to 2 to 5.5 volts.

So with maybe a little parts massaging you could use a 3.7 battery. Though pricey and needing a connection could you use two of these or something similar to get the life you desire? http://www.newark.com/dantona-industries/xb10/lithium-ion-battery-3-7v-550mah/dp/14N3902 They are thinner than a 9 volt battery so two could fit where one 9 volt battery currently resides.

[andybarrett1]

Both the RTC and LCD are 5V devices - RTC min is 4.5V, LCD min is 4.7 V.
So you really can't run your circuit at 3.3 V.
You will either need to use Ioannis' suggestion (which I also recommend), or go back to your 2936 regulator with at least a 6V battery. The 9V batteries have pretty low capacity - you might be better off with a couple 3V button cells; I have not worked out the details on those.

Hi yes the present circuit is built around a 5v supply....BUT :-

Looking to upgrade using LF818/19, LCD 3.3v and different RTC ...1307 has lousy timekeeping :-(

Application remains same... just maybe different parts!

BR
Andy

[Charlie]

Hi yes the present circuit is built around a 5v supply....BUT :-

Looking to upgrade using LF818/19, LCD 3.3v and different RTC ...1307 has lousy timekeeping :-(

Application remains same... just maybe different parts!

BR
Andy You'll need to choose those parts to know your supply tolerance. The answers are all in the data sheets.

[andybarrett1]

Thank you for reading

I know the info is on the datasheets... But. I did ask if anyone had suggestions or had previous examples / possibilities ??

Then I can choose some part numbers..... Have got some on way from ST and Microchip based on previous posts :-)

BR
Andy